1. Field of the Invention
This invention relates to a reading device to be used in a handling apparatus such as a facsimile, a copying apparatus, and so forth, as examples, and more particularly to a reading device having a function for selecting a reference white plate to make shading correction data suitable to a reading condition so as to avoid an uneven image output quality.
2. Discussion of the Background
A reading device for reading an original document has been used in several kinds of handling apparatus, such as, a facsimile machine, a copying machine, a scanner, and so forth, as examples. One type of reading system for a reading device is commonly referred to as a book scanning system in which an original document to be read, which original document may be in cut-sheet form and a book, is stationed on a supporting table. The original document is read by a reading unit which is movably supported by a frame underneath the supporting table in the reading device. This operation may be referred to as a moving reading mode. Another type of reading system, which may be referred to as a stationary reading mode, for a reading device is also commonly referred to as a sheet through scanning system in which only an original document in a sheet form is accepted, and such an original document is transferred so as to pass through a reading unit which is fixed underneath the supporting table in the reading device to read the sheets of such a fed original document. These two types of reading systems have different advantages. For example, the book scanning system has superiority in the range of reading subjects, from a sheet of a paper of various sizes and thickness to a book, a reading accuracy, and so forth, and, for example, the sheet through scanning system has superiority in a compactness, an inexpensiveness, and so forth. A reading device has been also introduced that implements both the book scanning system and the sheet through scanning system.
Generally, the above-mentioned reading device has an electrical correction process of a signal, a so called shading correction, to be applied when an image signal is generated by reading an original document. The shading correction corrects for an uneven white level of an image signal representing image information from the original document.
For this purpose of the shading correction, a reference white plate is provided in the reading device to provide a reference white level relative to a white level in the above-mentioned image signal. Such a reference white plate is scanned by the reading device before a start of reading information of an original document, and a reference white density is read as a result. FIGS. 11(a) and 11(b) are illustrations of signal waveforms in a full scanning line explaining an idea of the shading correction, and representing a reading of a reference white plate and a corrected waveform by the shading correction respectively. As shown in FIG. 11(a), V(p) of a peak value of a white density level in a reference white plate reading signal in a full scanning line is read in and is used as a reference value V(r) to correct a white density level in the reference white plate reading signal in a full scanning line as shown in FIG. 11(b).
A reading of a reference white plate is essential to ensure that an image signal is corrected by a superior shading correction with such a reference white plate. Various proposals related to this field have been introduced, accordingly. Some of these proposals, such as, for example, a shading distortion correction and a correction based on a reference brightness signal, are disclosed in the official gazette for Laid Patent Applications TOKUKAI HEI 2-134068, TOKUKAI HEI 2-149068.
However, such a shading correction system utilizing a single reference white plate suffers from drawbacks. More particularly, as a result of the number of original documents passing near the reference white plate in the sheet through scanning system, a scanning path to and a reading surface of the reference white plate may become dirty with dust after long use.
FIGS. 12(a) and 12(b) are illustrations of signal waveforms respectively representing an abnormal reading of a reference white plate which is dirty with dust and a correction result of the abnormal reading. As shown in FIG. 12(a), when a scanning path to or a reading surface of a reference white plate becomes dirty with dust, an output level of a reading of the reference white plate is correspondingly reduced at those dirty points as indicated by points A in FIG. 12(a). In this case, if a shading correction is performed on the signal of the above-mentioned abnormal reading of the reference white plate, and a reading of an original document is proceeded based on such a reference white level with an abnormal signal, the above-mentioned reduced output level indicated by points A in FIG. 12(a) correspondingly appears as peak values above a reference level in an output of reading the original document as indicated by spikes B in FIG. 12(b). This kind of an abnormal shading correction causes an abnormal image output, such as, for example, a vertical black line, a darkened image, or the like.
A degree of the above-mentioned dirtiness with dust around a scanning path to and a reading surface of a reference white plate is related to a size of a space between the reference white plate and an original document reading device thereof. Such a space may be made relatively small when an original document reading device having a high reading resolution or another original document reading device having a low illumination depth, such as a contact image sensor, is applied, although this does not totally overcome such problems, and may add expense.
As another background art, an image reading apparatus 2 as disclosed in Japanese Laid Open Patent Application 2-135068 as shown in FIG. 13 is now discussed. Such an image reading apparatus 2 includes an automatic document feeder 3, a control unit 21, a charge couple device (CCD) 22, a driver 23 for the CCD 22, a light source 24, a reflector 25, a lens 26, a glass platen 27, a reference position indicator for a book scanning operation 28, an original document holder 31, an exit path 32 and a reference position indicator for a sheet scanning operation 33. In this device as shown in FIG. 13, a reference white plate (not shown) can be utilized at a place where such a reference white plate can be read.
FIGS. 4(a)-4(c) are illustrations which explain characteristics of a light source, such as light source 24 of FIG. 13, in which a light amount is reduced over time, and which thereby similarly reduces an output level of read images and on corresponding copies if uncompensated. That is, as shown in FIG. 4(a), with a light source, such as light source 24 of FIG. 13, the amount of light decreases with the number of copies made, which thereby results in a corresponding reduction in an output level of the CCD 22, see FIG. 4(b). As a result, the corresponding sheets have an increased amount of background dirt, i.e., the sheets become less white, see FIG. 4(c).
In the device shown in FIG. 13, a reference white plate is read a plural number of times with changing a reading line thereon, before a start of reading an original document. A light amount at each picture element in a read line is then compared with detected light amounts of picture elements in other lines, and a picture element which has a maximum light amount among the compared lines is selected to be a part of the reference white data. Reference white data that covers a full scanning width of an original document is then constituted by the data of the selected picture elements, and is then used for a shading distortion correction. One object of such an arrangement is to reduce an opportunity of a data distortion caused by factors such as, for example, dust which adheres to or scratches a surface of the reference white plate, or an uneven light amount generated by light source 24.
However, such a device as disclosed in FIG. 13 suffers from significant drawbacks. In such a device, since only a single reference white plate is provided to be mainly used for the sheet scanning mode, and may be located at a position where the reference white plate is exposed to dust, a distortion on data is unavoidable if a distortion factor such as, for example, dust adhered to or a scratch formed on the surface of the reference white plate, or an unevenly generated light amount, is larger than the area across the plural lines being read by the image scanner. Or, when such a single reference white plate is provided to be mainly used for the book scanning mode, the single reference white plate may be positioned at another position where the single reference white plate is maintained at a relative difference from the reading position in the sheet scanning mode. In this case, when a plural number of original document sheets are read in the sheet scanning mode, to read the reference white plate before a start of reading each sheet is not practical, and therefore the reading of the first sheet is usually the reference white data for all subsequent sheets of the original document to be read. As a result, an output image density of the background becomes darkened from page to page, as shown in FIG. 4, because a relationship between the content reference white value and a reduction of the light amount over time reduces an output level from the CCD 22.
As shown in FIG. 4, and as described above, a first copy sheet is output with a clean background but a sixth copy sheet is output with a dirty background if the reference white data stored during the first sheet reading operation is used as the reference data. In a text image reading mode comparable to a halftone image reading mode, the text image is processed to binary data without referring to the reference white, and therefore an output image is less influenced by a reduction of an output level over time.